Organic electroluminescent (EL) devices (also known as organic light-emitting devices, organic light-emitting diodes, or OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED comprises, in sequence, an anode, an organic EL medium, and a cathode. The organic EL medium disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al. demonstrated highly efficient OLEDs using such a layer structure in “Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987) and in commonly assigned U.S. Pat. No. 4,769,292. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of Doped Organic Thin Films”, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly consists of a host material doped with a guest material. Further, there are other multilayer OLEDs that contain more functional layers in the devices. At the same time, many different types of EL materials are also synthesized and used in OLEDs. These new structures and new materials have further resulted in improved device performance.
Operational stability, and operational lifetime, of OLEDs is very important for display applications. The operational lifetime is defined as the time to reach half the initial luminance at a given current density. In order to improve operational stability, many different types of methods have been applied to fabricate OLEDs. For example, using a copper phthalocynine (CuPc) interfacial layer in between an anode and an HTL can improve the operational stability, as reported by Van Slyke et al. in “Organic Electroluminescent Devices with Improved Stability”, Applied Physics Letters, 69, 2160 (1996); an OLED containing a doped LEL can have improved operational stability, as reported by Hamada et al. in “Influence of the Emission Site on the Running Durability of Organic Electroluminescent Devices”, Japanese Journal of Applied Physics, 34, L824 (1995) and by Shi et al. in “Doped Organic Electroluminescent Devices with Improved Stability”, Applied Physics Letters, 70, 1665 (1997); an OLED containing a mixed LEL comprising both a hole-transporting material and an electron-transporting material can also have substantial improvement in the operational stability, as reported by Choong et al. in “Organic Light-Emitting Diodes with a Bipolar Transport Layer”, Applied Physics Letters, 75, 172 (1999) and by Aziz et al. in “Organic Light Emitting Devices with Enhanced Operational Stability at Elevated Temperatures”, Applied Physics Letters, 81, 370 (2002).
Currently, OLEDs with red or green emission have better operational stability than that of OLEDs with blue emission (or blue OLEDs). Therefore, improving the operational stability of blue OLEDs will have more impact on device applications. There are several ways to improve the blue emission stability. For example, Shi et al. in “Anthracene Derivatives for Stable Blue-Emitting Organic Electroluminescence Devices”, Applied Physics Letters, 80, 3201 (2002) and Hosokawa et al. in U.S. patent application 2003/0077480 A1, achieved improved operational stability of blue emission by selecting the appropriate materials. Other new methods for the improvement of the blue emission stability are certainly necessary.
As is known, low work-function metals can be used to dope into an electron-injecting layer (EIL) or ETL which is adjacent to a cathode to improve electron-injection and transport in an OLED. For example, Kido et al. reported in “Bright Organic Electroluminescent Devices Having a Metal-Doped Electron-Injecting Layer”, Applied Physics Letters, 73, 2866 (1998) and disclosed in U.S. Pat. No. 6,013,384 that an OLED can be fabricated containing a low work-function metal-doped electron-injecting layer adjacent to a cathode. These OLEDs containing a lithium (Li)-doped electron-injecting layer exhibit high luminous efficiency and low drive voltage. However, Li and other metals are reported as being diffusive in organic layer resulting in luminescence quenching in OLEDs, for example, as reported by Haskal et al. in “Lithium-Aluminum Contacts for Organic Light-Emitting Devices”, Applied Physics Letters, 71, 1151 (1997). An OLED containing a Li-doped tris(8-hydroxyquinoline)aluminum (Alq) layer would also face the problems of luminescence quenching and lifetime shortening, as discussed by Nakamura et al. in U.S. Pat. No. 6,509,109 B1.